The purpose of the current project is to improve our understanding how and to what extent higher-order component skills of reading (vocabulary, background knowledge, inferencing and strategies) and writing (complexity, organization, productivity, revision and editing) interact, and how they relate to changes in reading comprehension and writing across late elementary and middle grades. We focus on children with learning disabilities (LDs) as identified by dimensional and non-dimensional approaches and compare with typically developing students. This project has three specific aims. Aim 1 (reading comprehension) builds on experimental literature and evaluates the direct and mediated directional relations among components of reading (e.g., inferencing and background knowledge) and reading comprehension across grades 3-8. Historically, research on reading has mismatched the approach (reading comprehension as an outcome) and framework (reading comprehension as a system), employing methods that essentially treat reading comprehension as a unidimensional outcome rather than a complex system as described by cognitive models of reading. Aim 1 draws on data collected from a state agency to fit structural equation models (SEM) using item-level data to derive reliable sub-scales of higher-order component skills. This approach is particularly useful as it allows the focus to shift from reading as an outcome to a complex set of processes that influence change in outcome. The data for this phase of the project come from students (N = 42,095) in grades 3-8 in Connecticut (collected in 2008-2013). Aim 2 (writing) evaluates the development of components of writing and how they relate to written expression in typically developing, and children with LD. While research on writing has historically treated written expression as a multi-dimensional system, studies relating component skills and writing are often not informed by theories that specify the nature of their direct and mediated relations. In this phase of the project, we will analyze essays (N = 6,000) to obtain indices of writing quality/productivity using automated scoring to determine how writing develops as a function of higher-order processes of writing (e.g., revision, editing, organization). Aim 3 (co-development) examines how reading interacts with writing over time by assessing both longitudinally, for different components of reading and writing, different types of writing outcomes, and for students with and without LD. There is insufficient research on the development of higher- order components of reading and writing in typically developing and children with LD. Results will expand knowledge about direct and mediated relations among components in novel ways (e.g., across reading and writing domains), their effect on academic achievement at the same time-point and longitudinal development across elementary and middle school. The results will help to better characterize the reading and writing difficulties of students with LD.